This application is based on Korean Application No. 2000-32515 filed in the Korean Industrial Property Office on Jun. 13, 2000, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a lithium battery, and more specifically, to a lithium battery with an electrode evenly coated with active materials.
2. Description of the Related Art
Lithium-sulfur batteries use sulfur-based compounds as a positive active material and lithium metal, its alloys, or carbon-based compounds as a negative active material. In the negative electrode, the lithium metal is plated on charge and is reoxidized to lithium ion on discharge. The carbon-based compounds are compounds into or from which alkaline metal such as lithium or metal ion, such a lithium ion, is intercalated or deintercalated. The lithium-sulfur battery uses a material dissolving polysulfide as an electrolyte. In the positive electrode, on discharge, sulfur-sulfur bonds in the sulfur-based compound are cleaved by the electrolytic reduction to decrease the oxidation number of S. On charging, the inherent sulfur-sulfur bonds are reformed by electrolytic oxidation to increase the oxidation number of S. The electrical energy is stored in the battery cell as the chemical energy during charging and it is converted back to electrical energy during discharging.
Lithium is useful and attractive as an active material in an organic electrolyte battery because it has high energy density and good rechargeability.
A lithium negative electrode may be made of either the lithium metal itself, since the lithium metal serves as an active material as well as a current collector, or coating the lithium metal on a metal current collector substrate such as Cu or Ni. Such current collectors are often used since these metals have a much better mechanical strength than the lithium metal itself. The coating may be accomplished by vapor depositing evaporated lithium onto a current collector substrate for a desired thickness, or by pressing a lithium metal foil onto another metal foil such as Cu or Ni or a perforated current collector substrate such as an expanded metal sheet.
However, the vapor deposition process is expensive, and is difficult to control the process. In addition, the pressing techniques often result in weak adhesion, and it s also difficult to control the process. Also, since the lithium metal foil is soft, it easily forms an uneven film during the pressing process. Further, the lithium metal is extremely reactive with water, which causes the lithium to readily react with ambient moisture forming an undesirable surface film of an oxide or hydroxide. Such surface film interferes with a good adhesion between the current collector and the active material. In addition, such film at the lithium electrode surface interferes with a uniform electrode reaction throughout the surface of the electrode, thereby deteriorating the battery""s performance and life cycle characteristics.
It is an object of the present invention to provide a lithium battery including a stable negative electrode in which lithium metal is uniformly coated on a current collector substrate.
It is another object of the present invention to provide a lithium battery having a lithium electrode welded to a current collector substrate without an oxide film in between, by securely attaching a lithium metal onto the current collector through a simple process.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
In order to achieve the above and other objects, a lithium battery includes a lithium negative electrode, a positive electrode, a separator placed between the negative and the positive electrodes, and an electrolyte soaked into an assembly of the negative and the positive electrodes and the separator, the negative electrode is prepared by melting a lithium metal under a gas atmosphere to prepare a liquid lithium metal and coating the liquid lithium metal on a current collector, the positive electrode includes an electrically conductive metal and a binder, and any positive active material which is compatible with the lithium negative electrode.
According to an aspect of the invention, the positive active material includes at least one sulfur-based material selected from elemental sulfur, solid Li2Sn (nxe2x89xa71), and Li2Sn (nxe2x89xa71) dissolved in the catholyte.
According to another aspect of the invention, the electrolyte includes a lithium salt and an organic solvent or a mixture of organic solvents.
Reference will now be made in detail to the present preferred embodiments of the present invention.
A lithium battery of the present invention uses lithium metal as a negative active material. The lithium metal has high energy density and good rechargeability. The negative electrode preparation is illustrated in detail below. The lithium metal is melted under inert gas atmosphere to prepare a liquid lithium metal. The inert gas atmosphere is preferably argon (Ar) and free of moisture and oxygen. If the melting process is carried out under an atmosphere of the moisture and oxygen, it may cause the lithium metal and surface of a current collector substrate metal to react with moisture and oxygen, preventing good adhesion of the lithium on the current collector substrate. The lithium melting process may be performed by using a general metal melting process. For instance, the lithium metal is heated at 190 to 250xc2x0 C. under an inert gas atmosphere to melt the lithium metal to obtain the liquid lithium metal.
The liquid lithium metal is coated on a metal current collector substrate to a desired thickness. A coating process can be any general coating process. For instance, the coating process can be performed with either a doctor blade or by using a calendering process with a roller as the liquid metal is uniformly coated on the surface of the pre-heated current collector substrate. The metal current collector may be nickel or copper, or a metal-sprayed nickel or copper. The metal includes lithium-wetting metals such as Al, Si, Sn and other lithium-wetting metals or alloys. According to the coating steps, the liquid lithium metal is strongly and uniformly attached to the current collector. As a result, a high performance lithium electrode is fabricated for an improved battery performance.
A positive electrode may be any rechargeable positive electrode which is compatible with the lithium negative electrode. For example, the positive electrode is a positive active material using elemental sulfur, solid Li2Sn (nxe2x89xa71), and Li2Sn (nxe2x89xa71) dissolved in catholyte. In addition, at least one of organo-sulfur or carbon-sulfur polymer ((C2Sx)n, wherein, x=2.5xe2x88x9250, nxe2x89xa72) may be used. In order to prepare a positive electrode, a binder is dissolved in an organic solvent to prepare a binder solution, and an electrically conductive material is added to the solution and dispersed. Then, the positive active material is added to the resulting material to prepare a positive active material slurry. The positive active material slurry is coated on a current collector substrate to prepare the positive electrode.
For example, as the current collector, a carbon coated aluminum substrate may be used. The electrically conductive material may include carbon, carbon black, other electrically conductive compounds, or a mixture thereof. The binder may include polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl acetate, polyethylene oxide, polypyrrolidone, or polyvinyl alcohol. The organic solvent may be any organic solvent known in the related art, such as acrylonitrile.
An electrolyte includes a lithium salt and an organic solvent or solvents. The organic solvent may be any conventional organic solvent known in the related art such as 1,3-dioxolane, diglyme, sulforane, dimethoxy ethane, or a mixture thereof. The lithium salt may be any lithium salt known in the related art, such as LiSO3CF3, lithium triflate, lithium perchlorate, LiPF6 or LiBF4.
Hereinafter, the present invention will be explained in detail with reference to the examples. These examples, however, should not in any sense be interpreted as limiting the scope of the present invention.